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From: Wang Shanker <>
To: Ming Lei <>
	Xiao Ni <>
Subject: Re: [Bug Report] Discard bios cannot be correctly merged in blk-mq
Date: Wed, 9 Jun 2021 10:40:27 +0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <YMAOO3XjOUl2IG+4@T590>

> 2021年06月09日 08:41,Ming Lei <> 写道:
> On Tue, Jun 08, 2021 at 11:49:04PM +0800, Wang Shanker wrote:
>> Actually, what are received by the nvme controller are discard requests
>> with 128 segments of 4k, instead of one segment of 512k.
> Right, I am just wondering if this way makes a difference wrt. single
> range/segment discard request from device viewpoint, but anyway it is
> better to send less segment.
It would be meaningful if more than queue_max_discard_segments() bio's
are sent and merged into big segments. 
>>>> Similarly, the problem with scsi devices can be emulated using the following 
>>>> options for qemu:
>>>>       -device virtio-scsi,id=scsi \
>>>>       -device scsi-hd,drive=nvme1,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME1 \
>>>>       -device scsi-hd,drive=nvme2,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME2 \
>>>>       -device scsi-hd,drive=nvme3,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME3 \
>>>>       -trace scsi_disk_emulate_command_UNMAP,file=scsitrace.log
>>>> Despite the discovery, I cannot come up with a proper fix of this issue due
>>>> to my lack of familiarity of the block subsystem. I expect your kind feedback
>>>> on this. Thanks in advance.
>>> In the above setting and raid456 test, I observe that rq->nr_phys_segments can
>>> reach 128, but queue_max_discard_segments() reports 256. So discard
>>> request size can be 512KB, which is the max size when you run 1MB discard on
>>> raid456. However, if the discard length on raid456 is increased, the
>>> current way will become inefficient.
>> Exactly. 
>> I suggest that bio's can be merged and be calculated as one segment if they are
>> contiguous and contain no data.
> Fine.
>> And I also discovered later that, even normal long write requests, e.g.
>> a 10m write, will be split into 4k bio's. The maximum number of bio's which can 
>> be merged into one request is limited by queue_max_segments, regardless
>> of whether those bio's are contiguous. In my test environment, for scsi devices,
>> queue_max_segments can be 254, which means about 1m size of requests. For nvme
>> devices(e.g. Intel DC P4610), queue_max_segments is only 33 since their mdts is 5,
>> which results in only 132k of requests. 
> Here what matters is queue_max_discard_segments().
Here I was considering normal write/read bio's, since I first took it for granted
that normal write/read IOs would be optimal in raid456, and finally discovered
that those 4k IOs can only be merged into not-so-big requests.
>> So, I would also suggest that raid456 should be improved to issue bigger bio's to
>> underlying drives.
> Right, that should be root solution.
> Cc Xiao, I remembered that he worked on this area.

Many thanks for looking into this issue.


Miao Wang

  reply	other threads:[~2021-06-09  2:41 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-05 20:54 Wang Shanker
2021-06-05 22:38 ` antlists
2021-06-06  3:44   ` Wang Shanker
2021-06-07 13:07 ` Ming Lei
2021-06-08 15:49   ` Wang Shanker
2021-06-09  0:41     ` Ming Lei
2021-06-09  2:40       ` Wang Shanker [this message]
2021-06-09  8:44         ` Xiao Ni
2021-06-09  9:03           ` Wang Shanker
2021-06-18  6:28             ` Wang Shanker
2021-06-18 12:49               ` Xiao Ni
2021-06-21  7:49                 ` Wang Shanker
2021-06-22  1:48                   ` Xiao Ni

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